Such typical Cr--Ni martensitic stainless steels as ASTMA296 296 steel and CA6NM steel are widely used in such products as valves or runners which are required to exhibit sufficient levels of strength and corrosion resistance. In recent years, CA6NM steel and forged materials based thereon have also found applications in petroleum drilling equipment. On the other hand, austenitic stainless steel has an excellent corrosion resistance but it has a high susceptibility to chloride stress corrosion cracking. Accordingly, austenitic stainless steel cannot be used in cases involving a risk of crack formation due to stress corrosion and, if such is the case, chromium-based stainless steels or high Ni alloys are usually used. In particular, 410 steel and 430 steel, which are inexpensive, are widely used in such cases.
However, the corrosion resistance of 410 steel and 430 steel is not necessarily sufficient. In addition, these types of steel are generally known to have a significant drawback, that is, their very poor low-temperature toughness. Further, since 430 steel is a type of ferritic stainless steel, it cannot be used for a member or part which is required to have a high strength.
On the other hand, a martensitic stainless steel containing a few wt % of Ni can have its strength varied within a relatively wide range by slightly adjusting its composition and altering heat treatment conditions. This stainless steel also has an advantage that it has a very excellent low-temperature toughness. Although the addition of Mo not more than 1% is done generally to this steel so as to improve its strength, toughness and corrosion resistance, it is Ni that plays an important role in improving these characteristics, i.e., strength, low-temperature toughness and general corrosion resistance. However, a problem is encountered with stainless steels in that, although the addition of Ni imparts excellent characteristics, it also increases to an extreme extent the susceptibility to stress corrosion cracking. The level of the susceptibility in a Ni-containing martensitic stainless steel corresponds to that of AISI304 steel. For this reason, the martensitic stainless steel of this type cannot enjoy as wide an application range as that of 410 steel or 430 steel, in spite of the fact that this steel is excellent in other characteristics such as strength, toughness, and resistance to general corrosion.